The spark you already know
Rub a balloon on your hair and the hair stands up and follows it. Shuffle across a carpet in winter and a doorknob bites you with a tiny spark. A thundercloud does the same trick on a colossal scale. Behind all of it is one property of matter that this whole track is about: electric charge. The study of charges that are sitting still — no currents yet — is called electrostatics.
Do a careful experiment. Two balloons both rubbed on hair push each other apart; but a rubbed balloon and the hair pull toward each other. So charged objects come in two kinds, with a simple rule: like charges repel, opposite charges attract. Benjamin Franklin named the two kinds positive and negative. The names are arbitrary labels — he could have said 'red' and 'blue' — but they stuck.
Where charge lives
Zoom into any material and you find atoms: a tiny dense nucleus of protons (positive) and neutrons (neutral), surrounded by a cloud of electrons (negative). A proton carries charge +e and an electron carries exactly -e — the same size, opposite sign. A normal atom has equal numbers of each, so it is electrically neutral. Charge is simply this built-in property of these particles.
Charge is quantized: it always comes as a whole-number multiple of the elementary charge e. The SI unit is the coulomb (C).
That equation hides a deep fact: you never find half an electron's worth of charge. Charge is quantized, always an integer times e. The coulomb is a huge unit on this scale — one coulomb is about 6.2\times10^{18} elementary charges, which is why in everyday electrostatics we usually deal in microcoulombs (1\ \mu\text{C}=10^{-6} C) or nanocoulombs.
Charge is conserved
That last point is a law, not a coincidence. Conservation of charge: in any isolated system the total electric charge stays constant. Charges can be separated, moved, or paired up, but the net amount is fixed. When you rub the balloon, +Q appears on the hair exactly as -Q appears on the balloon — the two always come out summing to the zero you started with.
This holds even in the most violent physics. When a photon converts into an electron-positron pair, a -e and a +e appear together, summing to zero. Charge conservation is one of the most rigorously tested laws in all of physics — no exception has ever been seen.
Conductors and insulators
Materials differ sharply in how freely charge moves through them. In a conductor — a metal, say — some electrons are not bound to any one atom but wander through the whole material; they respond instantly to any push. In an insulator — glass, rubber, dry air, most plastics — every electron is tightly held to its atom and can barely budge.
How things get charged
There are three ways to charge an object. Friction (rubbing) transfers electrons between two materials. Conduction (contact) charges a neutral object by touching it with an already-charged one, so they share the charge. And induction charges a conductor without touching it: bring a negative rod close, and the metal's free electrons flee to the far side, leaving the near side positive. Ground the far side briefly to drain those electrons, remove the ground, then the rod, and the object keeps a net positive charge.
Induction also explains the classic puzzle: why does a charged balloon stick to a neutral wall? The wall is an insulator, so its electrons cannot leave their atoms — but they can shift slightly. The balloon's charge nudges the wall's molecules so their near sides become oppositely charged (this slight shift is called polarization). Opposite-facing surfaces attract, and the balloon clings. Honest caveat: this attraction is weak and needs the two surfaces very close together.